It might help to distinguish between
a) a historiographic approach to defining schools: the
historian of economics referring to a particular school
would have to use - implicitly or explicitly - a set of
criteria to count an individual to a particular school or
not
b) a sociological approach: observation of the actual
symbolic practices of economists when they refer to schools
(to the extent that they do); sb would belong to school X if
they were regarded by others (and/or themselves) to do so
(allowing for the case where there is controversy on this
issue); one could operationalise this by investigating
invocations of expressions referring to school X empirically
a) and b) are not mutually exclusive, but arguably, most
historians of economics strive for a conceptual rather than
sociological definition. One should note that putting b)
into practice will rarely uncover a conceptually homogeneous
body of writings. This reflects the seeming curiosity that
whenever a particular school is probed for consistency
across individuals one tends to uncover significant
degrees of heterogeneity.
Matthias Klaes